Sulfonated Surfaces by Sulfur Dioxide Plasma Surface Treatment of Plasma Polymer Films

Full Paper

Sulfonated Surfaces by Sulfur Dioxide PlasmaSurface Treatment of Plasma Polymer Films

Kim S. Siow,* Leanne Britcher, Sunil Kumar, Hans J. Griesser

The preparation of thin film coatings with sulfonate and sulfate groups by plasma techniquesis not straightforward due to limited volatility of suitable process vapours. We report acombination of plasma polymerisation and plasma treatment, treating heptylamine (HA-SO2)and 1,7-octadiene (OD-SO2) plasma polymers withsulfur dioxide plasma. HA-SO2 and OD-SO2

plasma polymer surfaces exhibited different com-positions with the latter showing almost twicethe amount of sulfur, as well as polysulfonate.Ageing of HA-SO2 plasma polymer surfaces in airor saline solution led to the disappearance ofoligomers containing sulfonate and sulfate.Angle-dependent XPS analysis of HA-SO2 plasmapolymers suggested that sulfur containing groupsreptated into the plasma polymer upon storageafter preparation.

Introduction

It is well established that the surface composition of

biomaterials dictates, upon contact between a material or

device and a biological environment, ensuing biological

processes such as competitive protein adsorption from

complex media. Thus there has been a large body of

literature on the fabrication of materials surfaces with

various chemical compositions and the study of their bio-

interfacial interactions. Early interest in sulfur containing

compounds arose from the role played by the sulfate groups

of heparin in anti-thrombogenicity.[1–3] Sulfate groups are

also found in other glycosaminoglycans present in extra-

cellular matrices, such as chondroitin sulfate, and as a

charged ionic group it is likely to be an essential part in

defining biological intermolecular interactions, and, hence,

K. S. Siow, L. Britcher, S. Kumar, H. J. GriesserIan Wark Research Institute, University of South Australia,Mawson Lakes, SA 5095, AustraliaFax: 65-68737192.; E-mail: [email protected]

Plasma Process. Polym. 2009, 6, 583–592

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

also some interactions between biological molecules and

materials surfaces. Sulfated materials surfaces may there-

fore be useful biomaterials interfaces, enabling the study of

the role played by these groups in protein/surface

interactions and extracellular signalling. It may, however,

be easier to fabricate sulfonated surfaces, and the surface

forces and effects may be very similar due to the very

similar properties, such as pK, of sulfonate (C�SO3) and

sulfate (C�O�SO3) groups on polymer surfaces. Besides

biomaterials applications, sulfonate containing surfaces

have been investigated as potential proton exchange

membranes in fuel cell application.[4–6] SOx containing

organic surfaces have been produced by bulk polymerisa-

tion,[7–13] gas[14–20] or acid sulfonation,[21,22] self assembled

monolayers,[23–25] and photochemical grafting.[26] These

methods, however, often lead to deterioration of mechan-

ical properties,[12,13] or are limited to a small number of

suitable substrates.[24,25]

Plasma approaches may overcome these problems, as

well as providing reduced process effluent discharge.[27]

Unfortunately, however, while various other biomaterials

DOI: 10.1002/ppap.200950004 583

K. S. Siow, L. Britcher, S. Kumar, H. J. Griesser

584

surfaces have been produced by plasma techniques,[28] the

direct fabrication of sulfonated or sulfated plasma polymer

coatings is not straightforward, as suitable monomers have

insufficient volatility for use in conventional plasma

polymerization. Previous studies have used sulfur dioxide

(SO2)[2–4,29–33] and allyl phenyl sulfone[29] for producing

SOx-containing polymer surfaces. While allyl phenyl

sulfone has a wide process window, its low volatility is a

disadvantage.[29] There is also the question of whether

sulfones would, upon plasma processing, convert to

sulfonates and sulfates, and/or become abstracted as polar

groups often do. X-ray photoelectron spectroscopy (XPS)

analysis of the S2p signal on such surfaces showed at least

two different oxidation states, with binding energy (BE)

values centred around 163–165 and 167–169 eV, respec-

tively. The lower BE component is assignable to S in

‘‘neutral’’ environments such as S�S, S�C, S�H, and the

other component to higher oxidation states such as SO2, SO3

and SO4.[3,4,30,32] The latter three are difficult to separate and

identify with certainty, not least because of considerable

variations in reported BE values for these groups, as we

found when reviewing reports of XPS spectra recorded on

various S containing surfaces.[34]

Success of SO2 plasma treatment appears to depend on

the balance between etching and implantation of chemical

functionalities. This balance likely varies with the dis-

charge power, the pressure, the process gas flow defined by

the plasma reactor geometry, as well as the substrate.

Therefore, it is difficult to define and transfer procedures for

producing the desired surfaces. There is also the question of

changes to surface compositions upon storage (‘‘ageing’’),

which again may vary considerably with the plasma

conditions and the substrate. SO2 plasma treatments

carried out on different polymeric or carbon substrates

resulted in unstable sulfur-based functionalities that

were susceptible to post-plasma ageing.[4] Ageing can

be due to two processes: oxidation and reptation.[35,36]

The process of reptation is often small in plasma polymers

due to extensive crosslinking.[35,36] Thus, instead of SO2

plasma treating polymers, such as polyolefins and

hydrogels that undergo reptation motions, it may be

preferable to first deposit a plasma polymer coating that is

then subsequently surface-sulfonated using SO2 plasma

treatment.

We report here the SO2 plasma treatment of plasma

polymers (pps) deposited from the two monomers 1,7-

octadiene (OD) and n-heptylamine (HA). The latter was of

interest to study whether it would be possible to produce

surfaces containing different types of chemical groups.

Will the SO2 plasma treatment modify mostly the

hydrocarbon segments, or also react with amine and

hydroxyl surface groups? Comparison of these plasma

polymers might enable investigation and understanding

of how different surfaces react to SO2 plasma treatment,



towards the generation of more predictable SO2 plasma

treated surfaces.

Experimental Part

Materials

MEMC prime grade silicon (100) wafers were procured from Micro

Materials and Research Consultancy, Australia. The wafers were

single-sided polished and p-type doped with boron. Silicon wafer

pieces of 1 cm2 were cut from wafers and cleaned in Piranha

solution (H2O2/H2SO4) for 10 to 15 min. CAUTION: Piranha

solution is extremely corrosive and hazardous and must only

be used by appropriately trained researchers using appropriate

safety measures. Then, the silicon pieces were washed with

copious amounts of deionized (DI) water (Millipore Milli-Q

filtration system) with a conductivity less than 0.5mS � cm�1

and a surface tension of 72.8 mN �m�1 at 20 8C. HA and OD were

obtained from Sigma-Aldrich and Fluka Australia, respectively.

Analytical grade hydrogen peroxide (30%) and sulfuric acid (98%)

were procured from Chemsupply and Southern Cross, Australia,

respectively. SO2 gas (99.98% purity) was obtained from BOC

Australia.

Plasma Deposition

Plasma processing, both polymerization and treatment, was

carried out in a custom-built reactor described elsewhere.[37] The

reactor comprised a glass cylinder of diameter 170 mm and

360 mm length, an upper electrode of two brass rods with a cross

strut and a 100 mm copper disc as the lower electrode. The plasma

chamber was pumped down to a base pressure of 0.03 Torr. Using

air as the process gas at a pressure of 0.1 Torr, the silicon

substrates were plasma cleaned for 2 min at 50 W in order to

remove residual adsorbed hydrocarbons. The residual air was then

removed and monomer vapour was fed into the chamber via

stainless steel lines with its flow controlled by a needle valve. The

reactor was powered by a commercial radiofrequency generator

(Advanced Energy RFX600) capable of delivering variable power

levels up to 600 W at 13.56 MHz via an impedance matching

network. For plasma polymerization of HA and OD, the rf power

levels used were 50 W and 10 W, respectively. The ODpp required

a deposition time of 5 min, while the HApp required only 90 s to

achieve a thickness >10 nm as assessed by absence of an XPS

signal from the Si substrate. After plasma polymerization, SO2

plasma treatment was carried out on the pp coated substrates at a

pressure of 0.3�0.1 Torr and a power level of 30 W for 3 min.

Plasma treated samples were then left in the chamber for 5–

10 min prior to venting.

Analytical Techniques

X-Ray Photoelectron Spectroscopy

XPS analysis was carried out on a Kratos AXIS Ultra DLD

spectrometer (Kratos Analytical, UK) equipped with a monochro-

DOI: 10.1002/ppap.200950004

Sulfonated Surfaces by Sulfur Dioxide Plasma Surface Treatment . . .

matic Al Ka radiation source (hv¼1486.6 eV) operating at a power

of 150 W (15 kV�10 mA). The take-off angle was 908 to the sample

surface except for the angle-dependent measurements. The

pressure in the main chamber was <10�9 mbar during analysis.

XPS survey spectra were recorded over the energy range 0–1 100 eV

at a pass energy of 160 eV with 1.0 eV resolution. High resolution

C1s, S2p and N1s signals were recorded at a pass energy of 20 eV

with 0.1 eV resolution. No changes in the spectra or other evidence

of radiation damage were observed during the analysis of the

plasma polymers.

The linearity of the XPS binding energy scale was confirmed

and calibrated periodically. This calibration is required to ensure

that the binding energy values for other elements are correct

when calculated via setting the ‘‘neutral’’ C peak (i.e. C�C and

C�H components) to 285.0 eV and thereby determining the

charge shift. S2p, C1s, O1s and N1s signals were acquired for the

quantification of elemental compositions in the survey scans.

High resolution peaks of the elements were also recorded for

component fitting. The S2p signal was used rather than S2s; the

Kratos library accounts for the different angular asymmetry

factors for s and p orbitals.

Samples were stored in tissue culture polystyrene petri dishes

(which, being free of additives, avoid contamination issues on

storage) at ambient temperature for various ageing periods, from

1 to 15 d. To avoid accumulation of radiation damage, replicate

samples not previously analyzed were used for each analysis.

Component fitting of the high resolution C1s, S2p and N1s

signals was carried out with CasaXPS software version 2.3.12. The

full width half maximum (FWHM) values used for fitting the C1s

and S2p peaks varied between 0.9 and 1.8 eV. We observe generally

for plasma polymers that the N and other heteroatom signals have

a greater FWHM than the C1s signal; this may be due to

inhomogeneous broadening arising from a larger range of bond

angles accessible to atoms that have fewer covalent bonds than C.

Within a single peak, the same FWHM was used for all components

to maintain consistency and optimization for comparisons

between different samples.[38,39] Due to the variations in the

literature, the binding energies of different oxidation states of

sulfur, to be fitted in the high resolution peaks, were calibrated from

model compounds.[34] Based on this exercise, the following

binding energies were used for the different oxidation states of

sulfur compounds: sulfate SO4 (168.8�0.2 eV), sulfonate SO3

(168.4�0.2 eV), sulfone SO2 (168.1�0.2 eV) and sulfoxide SO

(167.5�0.2 eV).[34] The line shapes of the peaks were assumed to

be Gaussian-Lorentzian with a 30% Lorentzian components and a

Shirley-type background was used throughout the analysis. Since

the S2p peak is a doublet, the following curve fitting criteria were

also applied: the area under the S2p1/2 peak was 50% of the area

under the S2p3/2 peak and the BE difference between the two peaks

was set to 1.2 eV.[40,41]

Since XPS is used here to compare different treatments

and processes, precision of the instrument is more important

than its accuracy. Systematic errors are not included in this

study. It is reasonable to assume that the systematic errors

associated with curve fitting and instrumental settings remain

constant throughout this study. The random error of the atomic

concentrations measured by XPS is assumed to obey the following

guidelines:



i. I
f the atomic concentration is more than 5%, the uncertainty is
5% of the value.

ii. I
f the atomic concentration is between 1% and 5%, the
uncertainty is 10% of the value.

iii. I
f the atomic concentration is 1% or less, the uncertainty is 20%
and increasing with decreasing absolute values.

For the calculation of the ratios O/C, O/S, and S/C, the following

formula was used. In a typical division between two parameters

(A� EA) and (B� EB), the error of the quotient is:[42]

Equotient ¼A

B

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEAA

� �2

þ EBB

� �2s

ð1Þ

‘‘A’’ and ‘‘B’’ are the variables. ‘‘EA’’ and ‘‘EB’’ are the errors of

uncertainty associated with the variables which are independent

and random. The error of uncertainty is based on the above

mentioned guidelines. In the current context, the error of

uncertainty is also known as the ‘‘standard deviation (stdev /sm)’’.

Attenuated Total Reflectance- Infra Red Spectroscopy

(ATR-FTIR)

Plasma polymers were deposited onto a 458ZnSe crystal, which was

then placed into a Harrick single internal reflectance attachment.

Infrared spectra were recorded in the range 600 to 1 500 cm�1 over

500 scans at a resolution of 5 cm�1 using a Nicolet FTIR spectro-

meter fitted with a MCT-B detector.

Results and Discussions

XPS Analysis

Visual observations indicated that both OD and HA

produced uniform plasma polymer coatings, and their

appearance did not change upon SO2 plasma treatment.

Ageing studies of HA plasma polymer have been reported

earlier by others[43–45] and serve as a baseline for our

observations. The oxidation of octadiene plasma polymer

can reasonably be expected to be analogous to that of

hexane plasma polymer, which has been reported ear-

lier.[46,47] We did not carry out ageing studies with OD pp

because the SO2 plasma treatment may alter the oxidative

processes and will mask any oxygen uptake from the OD pp

by itself.

In this current study, XPS analysis showed a higher

extent of sulfur incorporation into the OD plasma polymer

compared with the HA pp (Table 1). Oxygen is also present;

this will be discussed below in terms of post-plasma ageing.

For the HApp, the N percentage is, after accounting for the

presence of S, close to that reported in previous studies of

HApp films deposited using the same plasma equip-

ment[37,44] and to the value prior to SO2 plasma treatment,

www.plasma-polymers.org 585


Table 1. Elemental compositions, in at.-%, of HA-SO2 and OD-SO2plasma polymers determined by XPS.

Sample Element

at.-%

C1s N1s O1s S2p

ODpp 98.4 0 1.6 0

OD-SO2 pp 64.2 0 27.6 8.2

HA pp 86.7 11.3 2.0 0

HA-SO2 pp 69.5 10.2 15.3 5.0

586

indicating that this treatment did not significantly ablate

surface amine groups. The nature of the S-containing

groups was investigated using high resolution scans of the

S2p peaks (Figure 1) and component fitting, using

constrained fitting procedures and BE reference values

obtained with model compounds.[34] Results from compo-

nent fitting of the S2p photoelectron peaks of Figure 1 are

tabulated in Table 2. Clearly, the results of SO2 plasma

treatment vary with the plasma polymer being treated. It is

evident from these results that the approach of SO2 plasma

treatment of these two plasma polymers was successful in

generating sulfur moieties with higher oxidation states.

The component fitting shows for the OD-SO2 plasma

polymer surface three types of S environments, one being

‘‘neutral’’ sulfur species (S�S, S�C and S�H) and two being

oxidized sulfur species. The observed binding energies

suggest that a significant fraction is S with four surrounding

oxygen atoms, and the signals can be assigned to both

sulfate and persulfate (C�O�O�SO3). Their respective

ranges of BE values are 164.0–1, 168.9–169.1 and 169.6–

7 eV. While the identity of S with a binding energy of 169.6–

7 eV cannot be verified independently as persulfate, as no

suitable reference data or reference compounds were

Figure 1. S2p peaks of HA-SO2 and OD-SO2 plasma polymers,recorded 1 d after fabrication.



found, this putative structure appears reasonable in order

to produce such a high BE value; an alternative assignment

would be polysulfate, but it is not clear how this should be

produced. The highest oxidation state of S for which XPS

data exists is SO4, and all reported binding energies of SO4 do

not exceed 169.0 eV (all values were calibrated against

‘‘neutral’’ C at 285.0 eV).[34] In the case of HA-SO2 plasma

polymer surface, the chemical groups present are different.

Again ‘‘neutral’’ S is observed, and the oxidized S exists in a

variety of groups. Due to the small BE difference, we have

not attempted to separate SO and SO2 components. We

emphasize that there is also considerable uncertainty in the

quantitative fitting of the SO4 component due to its close

proximity of SO3 and SO4. However, it is clear that oxidised S

species are produced in substantial amounts, and that

higher oxidation states appear on the OD-SO2 surface. Based

on the data listed in Table 2, 67% of the S in the OD-SO2

plasma polymer consisted of SO3 and SO4 moieties or higher

oxidation states, while the percentage of S in the SO3 and

SO4 states in the HA-SO2 plasma polymer was 59%. The

additional presence of persulfate groups in the OD-SO2

plasma polymer increased the O/S ratios to 3.36 (�0.24)

compared with the ratio obtained for the HA-SO2 plasma

polymer of 3.07 (�0.34).

Component fitted C1s peaks for the two SO2-treated

plasma polymers are shown in Figure 2, and fit results are

listed in Table 2. C in C�S and C�O�S bonds was expected to

give rise to a component at�286.5 eV; we did not attempt to

fit them separately in the absence of reliable reference data

and in light of the finite signal-to-noise ratio of these

spectra. The HA-SO2 plasma polymer surface was found to

have a higher intensity of the component located at a BE of

286.5 eV than the OD-SO2 plasma polymer; this can be

assigned to the presence of C�N bonds in the former. Other

bonds possibly associated with this component at 286.5 eV

are C�O�C and C�OH; we will return to this in the

discussion on ageing. Again due to the close proximity of

expected BE values,[48] it is not meaningful to fit for separate

C�S/C�O�S and C�O�C/C�O�H components. The per-

centage of the C¼O component in the HA-SO2 plasma

polymer was also higher than for the OD-SO2 plasma

polymer, primarily because of an additional contribution

from amide N�C¼O C due to post-plasma oxidation of the

HA plasma polymer.[44] In the case of the HA-SO2 plasma

polymer, SO2 plasma might also react with amine groups to

form sulfonamides. However, the binding energies of

sulfonamide S2p was reported to be 168.8 eV,[49] which

overlaps with the SO3 and SO4 signals. The presence of

sulfonamide groups was, instead, probed by FTIR spectra

(below).

As shown in Figure 3, the N1s signal of the HApp showed

a combination of amine and amide groups, with the latter

produced by post-plasma oxidation between fabrication

and analysis as shown by earlier studies of HApp films.[50,51]

DOI: 10.1002/ppap.200950004


Table 2. Component fitting of XPS C1s and S2p signals of SO2 plasma treated plasma polymers.

Sample Components of C1s & S2p peaks

at.%

C�C,

C�H

C�Na), C�O�C,

C�OH

N�C¼Oa),

C¼O

S�C, S�H,

S�S

SO, SO2, SO3, SO4 SO4�SO4

C�O�SO4

OD-SO2 pp 55 7 2.2 2.7 0 2.3 3.2

HA-SO2 pp 44.5 13.5 4.3 0.9 1.4 3.3 0

a)This component is only present in the HA-SO2 plasma polymer.

The N1s signal of the HA-SO2 plasma polymer showed the

emergence of components associated with protonated

amine (Nþ) and an increase in the component assignable to

amide (O¼C�N) after the SO2 plasma treatment. During the

SO2 plasma treatment, oxygen-containing radicals and ions

from the plasma could have oxidized amines to amides. The

emergence of protonated amines can be attributed to

proton transfer during the plasma process. The FWHM

values of the N1s signal for HA-SO2 were determined to be

>1.60 eV when fitting for two components (amide and

protonated amine) as shown in Figure 3. The question arose

whether a third component should be added, but the two

components made physical sense and were located at the

expected BE values. To test for an alternative assignment,

the N1s signal was also curve-fitted using the constraint of

setting the FWHM of the components to be the same as for

the components of the C1s signal. When this procedure was

employed, three N1s components could be fitted corre-

sponding to amide (399.7–9 eV[44,50]), protonated amine

(401.5–6 eV[50]) and urethanes or imides (400.5–8 eV[44]). It

is not clear how urethane and imide groups would be

produced by the SO2 plasma treatment, and there is

considerable uncertainty anyway in such fitting due to the

close proximity to amides. While we cannot rule out the

Figure 2. C1s peaks of OD-SO2 and HA-SO2 plasma polymers,recorded 1 d after fabrication.



possibility of urethanes and imides being present, we prefer

the alternative assignment to two components only, and

postulate that the increased FWHM is due to a higher range

of bond angles as the N atom is less constrained in its range

of bond angles compared with tetrahedrally bonded C. For

the HApp surface likewise the N1s signal shows broader

FWHM values.

The O1s peak is generally difficult to interpret due to the

lack of consistent reference data for possible components,

recorded from simple reference molecules. Only the

hydroxyl O is usually readily separable; other O compo-

nents are ambiguous, as discussed in an earlier study on

plasma polymers.[52] We therefore decided not to provide

possible mis-interpretations of O1s components. Moreover,

the expected information – O in SOx – would not provide

additional information to that gained from S2p, N1s and

C1s spectra.

FTIR Analysis

As shown in Figure 4, FTIR spectra of OD-SO2 plasma

polymers exhibited well defined absorption bands corre-

sponding to the symmetrical stretching mode of SO2 in SO3

or SO4 groups at 1 180 cm�1.[53–55] The asymmetrical stretch

Figure 3. N1s peaks of HA plasma polymers before and after SO2plasma treatment.



Figure 4. FTIR spectra of OD and OD-SO2 plasma polymers,recorded 1 d after fabrication.

588

mode may be unobservable, as it might be prevented by

dense crosslinking of the plasma polymer. Absorption

bands at 1 050 cm�1 and 880 cm�1 were assigned to acid

sulfate (HSO�4 ).[53,55] The presence of HSO�

4 moieties has also

been reported by Coen et al., who used SO2-H2O plasma.[4]

This assignment is different from the sulfate moieties,

which are generally observed at 1 380 and 1 190 cm�1.[55]

The FTIR spectra of HA-SO2 plasma polymer differ from OD-

SO2 plasma polymer in the region around 750–800 cm�1

and 1 300–1 400 cm�1(Figure 5). The shoulders observed at

1 310 cm�1 and 1 180 cm�1 are assignable to the asym-

metric and symmetric stretching modes of SO2 in

sulfonamide (�SO2�N�), respectively.[53,54] The bands at

750–800 cm�1 are assignable to primary and secondary

amines or C¼S of N�C¼S bonds.[53,54]

Analogous to the OD-SO2 plasma polymer, the absorption

bands of the HA-SO2 plasma polymer can be assigned to the

asymmetrical and symmetrical stretch modes of the sulfur-

oxygen structure in various forms such as SO4 or SO3.[53] The

slight deviations in the frequencies of the bands observed

Figure 5. FTIR spectra of HA and HA-SO2 plasma polymers,recorded 1 d after fabrication.



from those normally associated with SO4 and SO3 groups

are probably caused by the transformation from the gas

phase to the condensed phase.[55] Figure 5 also shows the

presence of an acid sulfate (HSO�4 ) band at 1 070 cm�1 in the

HA-SO2 plasma polymer.[53] Such H are likely candidates for

the protonation of amine groups observed above in XPS N1s

signals.

Air-Ageing of HA-SO2 Plasma Polymers

It is well known that plasma polymers can undergo changes

after removal from the plasma chamber for extended

periods of time, due to oxidation of trapped radicals by

atmospheric oxygen.[44,50] In order to characterize post-

plasma changes to the plasma polymer surfaces upon

storage, XPS data were collected on replicate samples stored

for various periods of time after fabrication. The ageing data

in Figure 6 obtained at a take-off angle of 908 shows that the

O/S ratio exhibits an increase during the first 10 d of air

ageing. The N/C and S/C ratios remain unchanged during

this period; thus, there is no loss, by volatilization, of

heteroatom containing groups, but there is some oxidation

of S containing groups upon exposure to air. These

observations suggest that sulfur is the main element that

changes (decreases) its concentration with air ageing. This

could be due to preferential reptation of S-containing

polymer segments into the ‘‘bulk’’ (that is, below the XPS

probe depth) of the plasma polymer, or due to volatilization,

though it is difficult to see how processes such as liberation

of SO2 from sulfonamide would decrease the S content

while leaving the O content unchanged except if one

invokes coincidental compensation by oxidation. To

investigate the possibility of reptative migration of

Figure 6. Elemental ratios determined by XPS for HA-SO2 plasmapolymer at different times of air ageing at a take-off angle of 908.

DOI: 10.1002/ppap.200950004


Figure 7. S2p signals recorded on HA-SO2 plasma polymer surfacesat different times of air ageing, at a take-off angle of 908. Thepeaks are normalized to the same intensity at 164 eV.

Figure 8. Fitted percentages of components in the S2p signals ofHA-SO2 plasma polymer at different times of air ageing, at a take-off angle of 908.

polymer segments containing sulfur moieties, angle-

dependent XPS (ADXPS) was carried out at take-off angles

of 158 and 908 to compare the concentrations of sulfur

present within the top�2�3 nm and 9–10 nm, respectively.

This technique allowed us to study the possible migration of

sulfur containing groups into the plasma polymer, a

migration driven by translational entropy caused by the

high density of sulfur groups on the surface.

The S/C ratios at 908 were measured to be approximately

0.061� 0.006 independent of ageing, while the S/C ratios at

158 registered a decrease from 0.072 (�0.008) to 0.039

(�0.004) from 3 d to 15 d of ageing. The observed reduction

of the S/C ratio at a take-off angle of 158 suggests that sulfur-

containing polymer segments either reptate into the

plasma polymer ‘‘bulk’’ or volatilize into the atmosphere.

If sulfur moieties had volatilized, the S/C ratios at a take off

angle of 908 would be expected to decrease to a measurable

extent. If the reptation of sulfur moieties occurs only within

the top 10 nm, the S/C ratios recorded at 908 would

maintain similar values during the ageing period, as it is

observed. Evidently, the polar groups are driven from the

immediate air interface but end up in near-surface regions;

it appears reasonable to assume that the high crosslink

density, typical of plasma polymers, prevents long-range

migration of segments. The available results suggest that

the reptation mechanism is operational.

High resolution S2p signals were used to study whether

this putative reptation mechanism would show any

discrimination between the various S-containing species.

The S2p signals are shown in Figure 7 and the fitted

intensities of components are plotted in Figure 8 as a

function of ageing time, along with the total S %. These

results suggest that the time dependence of the S-contain-

ing groups is governed by two phenomena. One is reptation,

which seems to reduce the concentration of all S-containing

groups in the top 2–3 nm with time, but appears to do so to a

higher extent for the groups with a higher oxidation state.

This is reasonable, as more polar groups are less favoured

energetically at a polymer/air interface; thus, there is a

greater interfacial energy drive for the reptation of polymer

segments carrying SOx groups compared with those

bearing ‘‘neutral’’ S containing groups. However, the data

show that with time, the proportion of oxidized S species

increases again. This can be assigned to oxidation; its effect

becomes more evident as reptation slows down. Remaining

sulfur groups continued to be oxidized, which resulted in an

increase of the SO2/SO3/SO4 components after the first

week of air ageing (Figure 8). Thus, there is evidence of some

post-plasma oxidation, although the extent is minor

compared with the oxidation of amine plasma poly-

mers.[44,50,56]

In the case of the O/C ratios, ADXPS analysis of HA-SO2

plasma polymer samples showed an increase from 0.25

(�0.02) to 0.37 (�0.03) at a take off angle of 158, while the



ratios at a take off angle of 908 are maintained at 0.27–0.28

(�0.02). This parallels the trend of the S/C ratios at these two

take off angles during the same period of ageing. These

observations suggest that the oxidation of the HA-SO2

plasma polymer was restricted to the top few nanometers

over this time period. The results from component fitting of

C1s signals of air-aged HA-SO2 plasma polymer samples

obtained at a take-off angle of 908 are shown in Figure 9.

These signals show the formation of C¼O and N�C¼O

groups in the HA-SO2 plasma polymer. Rapid oxidation

occurred within the first 7 d, but slowed down thereafter.



Figure 9. C1s signals recorded on HA-SO2 plasma polymer surfacesat different times of air ageing, at a take-off angle of 908.

590

This ageing behaviour is similar to the oxidation of amine

plasma polymers, where the post-plasma oxidation of

radicals on the carbon atoms dominated the reaction

pathway.[50,56] Again, however, the changes are smaller in

comparison with pure amine plasma polymers. The N/C

ratios observed at take-off angles of 908 and 158 did not

show any significant variation, with values of 0.15–0.16

(�0.01). Figure 10 shows component-fitted N1s photoelec-

tron peaks for the HA-SO2 samples air-aged up to 15 d. The

atomic percentage of amide and protonated amine remains

relatively stable at 7.1 (�0.4) and 3.1 (�0.3) at.-%,

respectively, during this period. There is no evidence of

additional oxidation, to produce species such as nitroso and

nitro groups.

Saline Solution Ageing of the HA-SO2 PlasmaPolymers

For applications in the biomaterials and bio-diagnostics

fields, the behaviour of materials surfaces when exposed to

Figure 10. N1s signals recorded on HA-SO2 plasma polymer sur-faces at different times of air ageing, at a take-off angle of 908.



saline aqueous solutions is an important aspect. The

presence of some low molecular weight oligomeric

components in HA plasma polymers, albeit without SO2

plasma treatment, has been demonstrated recently.[57]

Table 3 shows chemical composition data recorded on HA-

SO2 plasma polymer surfaces upon incubation in 0.15 M

NaCl for 1, 14 and 24 h. Comparison of the data in Table 3

with the data of the non-incubated plasma polymer in

Table 1 shows that within the first hour of immersion, the S

percentage drops by 50%, from 5.0 to 2.5. As shown in

Table 3, the subsequently recorded data are within

experimental uncertainty; thus, the effects caused by

immersion in saline occur predominantly within the first

hour. From the component fitted S2p signals shown in

Figure 11, we can infer that the components associated with

SO3 and SO4 groups were reduced from 2.8 (�0.3) at % to 0.6

(�0.1) at.-% within the first hour of incubation. By an

incubation time of 14 h, the percentage of the SO3 and SO4

groups reduced further to 0.4 (�0.1) at.-%. The other

components approximately maintained their composition

during the entire period of incubation. These observations

can be rationalized by postulating that lower molecular

weight fragments of the polymer bearing polar, oxidized S

species are extracted by the aqueous solution. The

attachment of SO3 and SO4 groups may be associated with

decreased local crosslinking, thus leading to preferential

extraction of material carrying such groups.

During saline incubation, the N/C ratio obtained at a take

off angle of 908 reduced from 0.15� 0.01 to 0.11 (�0.01) for

the HA-SO2 plasma polymer upon incubation for 24 h.

Results from component fitting of the N1s peaks of the HA-

SO2 plasma polymer during incubation in 0.15 M NaCl

solution are displayed in Figure 12. The protonated amine

component at 401.3–401.6 eV reduced from 3.2% (�0.2%) to

1.24% (�0.1%) after 14 h of saline incubation. At the same

time, the component associated with amide at 399.7–9 eV

remained at approximately 7.6% (�0.6%) throughout the

14 h of incubation in saline solution. Again, this preferential

loss of the more polar components could be rationalized by

higher solubility in aqueous solution of polymer molecules

Table 3. Elemental compositions (in at.-%) determined by XPS onHA-SO2 plasma polymer samples incubated for 1, 14 and 24 h in0.15 M NaCl aqueous solution.

Sample Element

at.-%

C1s N1s O1s S2p

HA-SO2 pp incubated 1 h 78.8 9.6 9.1 2.5



DOI: 10.1002/ppap.200950004


Figure 11. S2p signals recorded on HA-SO2 plasma polymer sur-faces at different times of ageing in 0.15 M NaCl (aq), at a take-offangle of 908.

Figure 12. N1s signals recorded on HA-SO2 plasma polymer sur-faces at different times of ageing in 0.15 M NaCl (aq), at a take-offangle of 908.

carrying more polar groups. During saline incubation of HA-

SO2 pp for 24 h, the O/C ratio reduced from 0.22 (�0.02) to

0.11 (�0.01). This reduction in the O/C ratio parallels the loss

of SO3 and SO4 groups.

Conclusion

The SO2 plasma surface treatment of heptylamine and 1,7-

octadiene plasma polymers was successful in producing

surfaces containing sulfonate and sulfate groups. The two

plasma polymers exhibited significantly different compo-

sitions after SO2 plasma surface treatment, with the latter

showing almost twice the amount of sulfur compared with

the former. FTIR spectra of OD-SO2 and HA-SO2 plasma



polymers showed bands assignable to characteristic SO2

stretching modes of SO3 or SO4. For the HA-SO2 surface,

bands assignable to sulfonamide were also present. Air

ageing and saline incubation of HA-SO2 plasma polymers

showed predominantly the disappearance of oligomers

containing SO3 and SO4 groups, by leaching and reptation.

Air ageing also caused the formation of C¼O and N�C¼O

groups in HA-SO2 plasma polymers as part of the oxidation

cycle, but this was significantly less so upon incubation in

saline solution. Angle dependent XPS analysis of HA-SO2

plasma polymers suggested that sulfur containing groups

reptated into the plasma polymer.

Acknowledgements: This work was supported by the AustralianResearch Council via the Special Research Centre for Particle andMaterial Interfaces, including a PhD scholarship for KSS.

Received: January 11, 2009; Revised: April 26, 2009; Accepted:May 12, 2009; DOI: 10.1002/ppap.200950004

Keywords: ageing; ESCA/XPS; heptylamine; octadiene; plasmapolymerization; plasma treatment; sulfate; sulfonate; sulfurdioxide

[1] J. C. Eriksson, C. G. Goelander, A. Baszkin, L. Ter-Minassian-Saraga, J. Colloid Interface Sci. 1984, 100, 381.

[2] J. Lahann, D. Klee, H. Thelen, H. Bienert, D. Vorwerk, H. Hocker,J. Mater. Sci.: Mater. Med. 1999, 10, 443.

[3] T. A. Giroux, S. L. Cooper, J. Appl. Polym. Sci. 1991, 43, 145.[4] M. Collaud Coen, B. Keller, P. Groening, L. Schlapbach, J. Appl.

Phys. 2002, 92, 5077.[5] Z. Ogumi, Y. Uchimoto, Z. Takehara, F. R. Foulkes,

J. Electrochem. Soc. 1990, 137, 29.[6] Z. Ogumi, Y. Uchimoto, Z. Takehara, J. Electrochem. Soc. 1990,

137, 3319.[7] A. Z. Okkema, S. A. Visser, S. L. Cooper, J. Biomed. Mater. Res.

1991, 25, 1371.[8] Y. H. Kim, D. K. Han, K. D. Park, S. H. Kim, Biomaterials 2003,

24, 2213.[9] T. G. Grasel, S. L. Cooper, J. Biomed. Mater. Res. 1989, 23, 311.

[10] J. P. Santerre, P. Ten Hove, N. H. VanderKamp, J. L. Brash,J. Biomed. Mater. Res. 1992, 26, 39.

[11] J. P. Santerre, P. T. Hove, J. L. Brash, J. Biomed. Mater. Res. 1992,26, 1003.

[12] J. H. Silver, J. W. Marchant, S. L. Cooper, J. Biomed. Mater. Res.1993, 27, 1443.

[13] R. W. Hergenrother, S. L. Cooper, J. Mater. Sci.: Mater. Med.1992, 3, 313.

[14] H. M. Kowalczynska, J. Kaminski, Colloids Surf. B: Biointer-faces 1994, 2, 291.

[15] H. M. Kowalczynska, Colloids Surf. B: Biointerfaces 1995, 4,137.

[16] J. Dobkowski, R. Kolos, J. Kaminski, H. M. Kowalczynska,J. Biomed. Mater. Res. 1999, 47, 234.

[17] H. M. Kowalczynska, M. Nowak-Wyrzykowska, J. Dobkowski,R. Kolos, J. Kaminski, A. Makowska-Cynka, E. Marciniak,J. Biomed. Mater. Res. 2002, 61, 260.



592

[18] H. M. Kowalczynska, M. Inkielman, M. Nowak-Wyrzykowska,L. Stolowska, J. Doroszewski, Colloids Surf. B: Biointerfaces2003, 30, 193.

[19] H. M. Kowalczynska, M. Nowak-Wyrzykowska, Cell Biol. Int.2003, 27, 101.

[20] H. M. Kowalczynska, M. Nowak-Wyrzykowska, R. Kolos, J.Dobkowski, J. Kaminski, J. Biomed. Mater. Res. Part A 2005,72A, 228.

[21] A. S. G. Curtis, J. V. Forrester, C. McInnes, F. Lawrie, J. Cell Biol.1983, 97, 1500.

[22] S. Kim, J. Yun, S. Lee, ‘‘Platelet Adhesion on Plasma-treatedPolyethylene Incorporating Sulfate and Phosphate Groups’’,in: Trans. 5th World Biomater. Congr, Toronto, Canada 1996,p. 408.

[23] J. H. Silver, J.-C. Lin, F. Lim, V. A. Tegoulia, M. K. Chaudhury,S. L. Cooper, Biomaterials 1999, 20, 1533.

[24] M. Lestelius, B. Liedberg, P. Tengvall, Langmuir 1997, 13, 5900.[25] J. C. Lin, W. H. Chuang, J. Biomed. Mater. Res. 2000, 51, 413.[26] T. Kavc, W. Kern, M. F. Ebel, R. Svagera, P. Poelt, Chem. Mater.

2000, 12, 1053.[27] H. Yasuda, Y. Matsuzawa, Plasma Process. Polym. 2005, 2, 507.[28] K. Siow, L. Britcher, S. Kumar, J. H. Griesser, Plasma Process.

Polym. 2006, 3, 392.[29] T. M. Ko, J. C. Lin, S. L. Cooper, Biomaterials 1993, 14, 657.[30] J. C. Lin, S. L. Cooper, Biomaterials 1995, 16, 1017.[31] J. C. Caro, U. Lappan, F. Simon, D. Pleul, K. Lunkwitz, Eur.

Polym. J. 1999, 35, 1149.[32] J. Gu, X. Yang, H. Zhu, Mater. Sci. Eng. C 2002, 20, 199.[33] Q. Lu, C. Cao, H. Zhu, J. Mater. Sci.: Mater. Med. 2004, 15, 607.[34] K. Siow, PhD thesis, University of South Australia, Mawson

Lakes 2007.[35] R. C. Chatelier, X. Xie, T. R. Gengenbach, H. J. Griesser, Lang-

muir 1995, 11, 2585.[36] R. C. Chatelier, X. Xie, T. R. Gengenbach, H. J. Griesser, Lang-

muir 1995, 11, 2576.[37] S. A. Al-Bataineh, L. G. Britcher, H. J. Griesser, Surf. Sci. 2006,

600, 952.[38] N. Fairley, ‘‘XPS Lineshapes and Curvefitting’’ in: Surface

Analysis by Auger and X-ray Photoelectron Spectroscopy, D.Briggs, J. T. Grant, Eds., IM Publications and SurfaceSpectraLimited, Chichester 2003, p. 397.



[39] N. Fairley, A. Carrick, ‘‘The Casa Cookbook: Part 1: Recipes forXPS Data Processing’’, Acolyte Sci, Cheshire 2005.

[40] B. J. Lindberg, K. Hamrin, G. Johansson, U. Gelius, A. Fahlman,C. Nordling, K. Siegbahn, Phys. Scr. 1970, 1, 286.

[41] G. Beamson, D. Briggs, ‘‘High Resolution XPS of Organic Poly-mers’’, Wiley, New York 1992.

[42] B. A. Barry, ‘‘Errors in Practical Measurement in Science,Engineering and Technology’’, Wiley-Interscience Publication,New York 1978.

[43] T. R. Gengenbach, Z. R. Vasic, S. Li, R. C. Chatelier, H. J. Griesser,Plasmas Polym. 1997, 2, 91.

[44] T. R. Gengenbach, R. C. Chatelier, H. J. Griesser, Surf. InterfaceAnal. 1996, 24, 271.

[45] R. C. Chatelier, C. J. Drummond, D. Y. C. Chan, Z. R. Vasic, T. R.Gengenbach, H. J. Griesser, Langmuir 1995, 11, 4122.

[46] T. R. Gengenbach, Z. R. Vasic, R. C. Chatelier, H. J. Griesser,J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1399.

[47] T. R. Gengenbach, Z. R. Vasic, R. C. Chatelier, H. J. Griesser,Plasmas Polym. 1996, 1, 207.

[48] N. Larsson, P. Stenius, J. C. Eriksson, R. Maripuu, B. Lindberg,J. Colloid Interface Sci. 1982, 90, 127.

[49] G. A. Schick, Z. Sun, Langmuir 1994, 10, 3105.[50] T. R. Gengenbach, H. J. Griesser, J. Polym. Sci., Part A: Polym.

Chem. 1999, 37, 2191.[51] A. Tarasova, P. Hamilton-Brown, T. R. Gengenbach, H. J.

Griesser, L. Meagher, Plasma Process. Polym. 2008, 5, 175.[52] T. R. Gengenbach, R. C. Chatelier, H. J. Griesser, Surf. Interface

Anal. 1996, 24, 611.[53] G. Socrates, ‘‘Infrared Characteristics Group Frequencies:

Tables and Charts’’, 2nd Ed. John Wiley & Sons, Chichester1994.

[54] L. Bellamy, ‘‘The Infrared Spectra of Complex Molecules’’,3rd Ed. Chapman and Hall, London 1975.

[55] D. Lin-Vien, B. N. Colthup, W. G. Fateley, J. G. Grassell,‘‘The Handbook of Infrared and Raman Characteristic Fre-quencies of Organic Molecules’’, Academic Press, San Diego1991.

[56] T. R. Gengenbach, X. Xie, R. C. Chatelier, H. J. Griesser, J. Adhes.Sci. Technol. 1994, 8, 305.

[57] K. Vasilev, L. Britcher, A. Casanal, H. J. Griesser, J. Phys. Chem. B2008, 112, 10915.

DOI: 10.1002/ppap.200950004

Sulfonated Surfaces by Sulfur Dioxide Plasma Surface Treatment of Plasma Polymer Films

Documents